There are many species of wood with different hardness. You'd have to specify which type of wood you would use to compare.
What is the difference in the density of wood compared to a human skull? If I were to impact them with the same amount of force which would be more resistant? Any idea would be helpful.
Well, as it was said different wood have different density. Some trees can't float, for instance. They drown immediately because they are so dense. Anyways, the question is strange and complicated. I've worked as a carpenter for quite a while. I have a wood router https://www.woodroutersreviews.com/ and many other tools I use to process wood. But even with such experience, I can't say it even approximately.